High-frequency bridge circuit



Dec. 25, 1951 R. B. MUCHMORE 2,579,751

HIGHFREQUENCY BRIDGE CIRCUIT Filed Nov. 26, 1948 pan-c701 29 INVENTORMom/1.41:0 J24 asc/LL4T0R U Patented Dec. 25, 1951 HIGH-FREQUENCY BRIDGECIRCUIT Robert B. Muchmore, Hawthorne, Calif., assignor to The SperryCorporation, Great Neck, N. Y., a corporation of Delaware ApplicationNovember 26, 1948, Serial No. 62,047

4 Claims. 1

This invention relates to improvements in radio and higher frequencybridge circuits, a bridge circuit being defined as a network havingconjugate pairs of terminals which are effectively isolated from eachother when the impedances connected to certain terminals are equal orbalanced.

Conventional Wheatstone bridge circuits and hybrid coils or transformersare unsatisfactory at high radio frequencies, particularly formeasurement purposes, owing to stray reactance and radiation from thebridge elements. Careful shielding is effective at moderately highfrequencies, but at the higher frequencies it is found that intolerablyhigh losses occur in lumped circuit networks with shields.

At the extremely high frequencies characteristic of the so-calledmicrowave portion of the electromagnetic spectrum, the functions ofbridge circuits may be effected conveniently by wave guide networks ofthe hybrid tee or magic tee type. At somewhat lower frequencies, whichare nevertheless too high for conventional bridge circuits to bepractical, wave guides must be undesirably large in order to be abovethe minimum size for low frequency cutoff. Transmission line networkssuch as rings or rat races are used at these frequencies, since a twoconductor line is not limited by a low frequency cutoff and can bereasonably small.

The usual prior art transmission line bridge circuit includes one ormore line sections whose length depends upon the frequency at which thebridge is to be used. At frequencies other than the design frequency,the line sections are too long ortoo short, and the impedances presentedby the bridge at its various terminals are not of the intended values.Another, and probably much more serious difliculty, is that the balancecharacteristics of the circuit depend upon the line lengths and thus thecircuit may not act like a bridge at all at frequencies not close to thedesign frequency.

One of the principal objects of the instant invention is to provideimproved radio frequency bridge circuits which may be of reasonablysmall physical size when designed for use in the comparatively longerwavelength part of the microwave spectrum, and lower frequencies.

Another important object is to provide bridge circuits wherein thebalance characteristics depend only upon mechanical symmetry and thusare independent of frequency.

A further object of this invention is to provide bridge circuits whichfulfill the foregoing objects and in addition are electrically andmechanically simple, rugged, and easy to construct with the requireddegree of symmetry.

The invention will be described with reference to the accompanyingdrawing wherein:

Fig. 1 is a perspective view of an enclosed transmission line bridgestructureembodying the invention;

Fig. 2 is a plan view in section of a modification of the structure ofFig. 1;

Fig. 2a is a perspective view of a part of the device of Fig. 2, withthe outer sheath broken to show internal details;

Fig. 3 is a schematic drawing of another modification; and

Fig. 4 is a schematic drawing of a further embodiment of the inventionin a reflectometer system.

The device shown in Fig. 1 includes two coaxial line sections II and 13in alignment with each other and with the near ends of their outerconductors or sheaths joined together and to the ends of two furthersimilar sheaths l5 and 11 at a common junction i9. An inner conductor 2|extends into the sheath [5 and through the junction of the sheaths intothe sheath H. The inner conductors of the line sections II and I3 extendaround the interior of the junction I9 and through the sheath I! asshown at 23 and 25 respectively, parallel to and on opposite sides ofthe conductor 2!. The inner conductor 2! may terminate at a point 4 inthe vicinity of the end of the sheath H.

The structure is physically symmetrical about a plane through the axisof the sheaths l5 and I! and perpendicular to that of the lines H and [Iand I3 perpendicular to both sheaths I5 and H, as shown in Fig. 1.

In a typical application the bridge device of Fig. 1, a source 24 ofradio or microwave energy is connected to the outer end 6 of thecoaxialline formed by the conductors l5 and 2|, and impedances 26 and 21are connected to the ends I and 2 of the lines I I and I3 respectively.A balanced detector 29 is connected-to the ends 3 3 and 5 of theconductors 23 and 25, near the end of the sheath [7.

The detector :29 may comprise a pair of diodes or crystals 3| and 33connected between the terminals 3 and 5 and the respective ends of acenter-tapped resistor 35. The center-tap of the resistor 35 isgrounded, and a direct current meter such as a galvanometer 31 isconnected across the resistor 35. The resistor and meter are shunted bya capacitor 39 'In the operation of the system of Fig. 1, energy fromthe source 2 3 travels down the line 2i, l5 toward the point 4. Some ofthis energy is transferred to each of the conductors 23 and 25 by way ofcapacitive coupling, the capacitance being distributed along theconductors 23 and 25, and

by way of inductive coupling, whereby the charging currents flowingalong the conductor 2! induce corresponding currents in the conductors23 and 25. One or the other type of coupling may predominate, dependingupon the wave-.

length and upon the dimensions of the various elements; regardless ofthe exact quantitative relationship,it will be apparent that as long asthe structure is physically symmetrical, and the inner conductors 23 and25 are terminated symmetrically at their respective ends, equal currentsof the same instantaneous polarities will be induced therein.

For this condition to exist, it is necessary not only that the detector29 be balanced with respect to ground, but also that the impedances Z1and Z2 of the elements 26 and '27 be identical. In this event, equalcurrents will flow through the rectifiers 3| and 33, producing equaldirect currents to ground in the two halves of the resistor 35. Thevoltage drops across the two halves of the resistor will be equal andopposed, and no current will flow in the meter 31. t

Now if either of the iinpedances Zi and'Zz' is changed, oranotherimpedance of different value substituted for one of them, the currentsinduced in the conductors 23 and 25 will no longer be equal. The outputsof the rectifiers, and hence the voltage drops on the two halves of theresistor 35, will differ. The meter 31 will show a deflection in onedirection or the other, indicating unbalance between the two impedancesconnected to the terminals l and 2.

It will be apparent that the bridge structure of s:

Fig; 1 may be used in various other circuit arrangements where balancecharacteristics are required. For example, a push pull oscillator,

balanced to ground, may be connected to the impedances so connectedwould produce output.

Thus, as in other types'of bridge circuits, the connections to variousconjugate terminals may be interchanged. It should be noted that in anyof the above mentioned arrangements, the balance characteristics dependonly upon the phy- V sical symmetry of the bridge device, and thus are 1not a function of frequency.

{The nature of the couplingbetween the con- 7 4 ductor 2| and theconductors 23 and 25 can be controlled to some extent by the terminationof the conductor 2 I. within the sheath I'I. Referring to Figs. 2 and 2afor example, the conductor 2! terminates within the sheath I! at atransverse conductor or fin M to which it is connected and which in turnis connected to the inner wall of the sheath, extending between andsymmetrically with respect to the conductors 23 and 25. A wire conductorcould be used in lieu of fin 41,

if desired. e

The operation of the device of Fig. 2 is like that of Fig. 1, exceptthat the mode of coupling may be diiferent, being principally inductiveor capacitive depending upon the frequency and the balance will besubstantially constant over said band. Thestructure shown schematicallyin Fig. '3

is like that of Fig. 1, except that the conductor ends at a point nearthe center of the junction. This arrangement also operates like that ofFig. 1 as far as balance is concerned, the principal difference beingthat the coupling in Fig. 3 is primarily capacitive. V

Fig. 4 shows a device similar to that of Fig. 3, but omitting the sheathll and the conductors 23 and 25. In this arrangement, a balanceddetector 29 may be connected within the junction between the ends 3' and5 of the inner conductors of'the lines H and 13 respectively. Thcoupling from the conductor 2! tothe lines H and I3 is principallycapacitive, and equal volt rent for the deteetor 2d is supplied throughthe center conductors from a batteryAB.

r It is assumed that the test load 21 has a D.-C. return path for thebias current. 24' may be square-wave modulated at a frequency of, forexample, 400 cycles per second. A resistor 4'5 serves to control thebias current and also acts as an audio frequency load, being coupledthrough a blocking condenser 49 to an A.-C.

, indicator, not shown.

When the test load 2? matches the line IS, the radio frequency voltagesat the terminals 3' and 5' are equal. The resistance of the deviceZBdepends upon the bias current supplied by the bat: tery 45, and does notvary] If the test load 21 does not match the line !3, the P..-F.voltages at the points 3' and 5' are unequal, and a radio frequencycomponent is superimposed on the bias current in the element 29. Sincethis current is modulated, the resistance will vary at the modulationfrequency] The current drawn from the battery will vary accordingly, aswill the volt-v age drop across the load resistor ii. 7 The resultinge00 cycle voltage will be coupled through the capacitor 43 to theindicator, and its amplitude will be ameasure enhancement mismatch be- 7The source tween the load 21 and the line It. A small axial by-passcondenser prevents any substantial amount of radio frequency energy fromreaching the indicator. Providing the matching device 43 is designed tobe effective over a band of frequencies, the standing wave ratio of thetest load 21 as a function of frequency may be determined by varying thecarrier frequency of the source 27.

What is claimed is:

1. A radio frequency bridge device, including two coaxial transmissionline sections with the proximate ends of their outer conductors abuttingand joined, a third outer conductor joined at the junction thereof tosaid first two outer conductors, the inner conductors of said firstmentioned lines being continued through said junction and in symmetricalrelationship to each other through said third outer conductor andparallel to the axis thereof, a further coaxial line with its outerconductor joining said other outer conductors at said junction, theinner conductor of said further coaxial line extending through saidjunction into said third outer conductor, and means shortcircuiting saidinner conductor to said third outer conductor.

2. The invention as set forth in claim 1, wherein said meansshort-circuiting said inner conductor to said third outer conductor isat a point which is of the order of one quarter wavelength beyond saidjunction.

3. A radio frequency bridge device, including two coaxial transmissionline sections having a common axis and the proximate ends of their outerconductors abutting and joined, a third outer conductor joined at thejunction thereof to said first two outer conductors and extending atright angles thereto from said junction, the inner conductors of saidfirst mentioned lines being continued through said junction and insymmetrical relationship to each other into and through said third outerconductor and parallel to the axis thereof, and a further coaxial linewith its outer conductor joining said other outer conductors at saidjunction and extending at right angles to said two first mentionedlines, the inner conductor of said further coaxial line extending intosaid junction midway between said inner conductors of said firstmentioned lines, and conductive means connecting said further innerconductor to said third outer conductor in the plane of symmetry betweensaid first and second inner conductors.

4. A radio frequency bridge device, including two coaxial transmissionline sections with the proximate ends of their outer conductors joined,a third outer conductor joined at the junction thereof to said first twoouter conductors, the inner conductors of said first mentioned linesextending through said junction and through said third outer conductorand parallel to the axis thereof, a further coaxial line with its outerconductor joined to said other outer conductors at said junction and itsinner conductor extending through said junction and into said thirdouter conductor along the axis thereof, and means including a member ofresistive material within said outer conductor connecting said lastmentioned inner conductor to said third outer conductor.

ROBERT B. MUCHMORE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,441,574 Jaynes May 18, 19482,445,895 Tyrrell July 27, 1948 2,454,907 Brown Nov. 30, 1948 2,456,679Cork et a1 Dec. 21, 1948 2,458,577 Evans Jan. 11, 1949 2,527,979Woodward Oct. 31, 1950

